The package system, where a plastic container such as bottle, cup or tray is sealed with lid comprising laminated plastic film or laminated aluminum foil, is widely used for packages such as snack package for rice crackers, potato chips, fluid package for jelly, milk, yogurt, pudding, tofu (soybean curd), lactic acid beverage, etc., blister package, and other package for medical drug, medical instruments, daily goods and miscellaneous goods. These packages are required for mechanical strength to withstand distribution channels, the sealing strength to keep their hygienic property, and easy peelability.
Packages in which films categorized as easily openable (easily strippable or easily peelable) films are used as the innermost heat-sealed layer of bags or lid materials have been widely utilized as one of the materials used for such packaging. While various heat-sealed layers proposed so far have a strong heat seal strength for the purpose of tight sealing, its easy peelability is insufficient. Therefore, it is required to strike a proper balance between the heat seal strength and the easy peelability. The characteristics required for the easy peelability herein are not only easy opening but also no residues of resin, which will be caused by a stringing phenomenon after opening in an unsealed region after it is unsealed, i.e., the overall evaluation, including good appearance of the unsealed region, thereby to judge whether good or bad.
Many proposals have been made hitherto for films having such an easy-openable property (easy peelability). For example, Japanese Patent Publication No. HEI 5(1993)-64593 proposes an easily openable package comprising a layer of polypropylene resin/polyethylene resin mixture as an easily peelable layer and a high density polyethylene resin layer. Japanese Patent Publication No. HEI 5(1993)-6513 proposes a sealant film for lid materials comprising an adhesive layer (easily peelable layer) composed of a resin mixture of polypropylene and polyethylene and a substrate layer (laminated layer). Furthermore, Japanese Patent Laid-Open Publication No. HEI 2(1990)-185547 proposes a resin composition for easily openable heat seal comprising an ethylene-α-olefin copolymer, a propylene-ethylene copolymer and low density polyethylene, and Japanese Patent Laid-Open Publication No. 2000-355358 proposes a multi-layer sheet having a sealed layer comprising a polypropylene resin, an ethylene-α-olefin random copolymer resin having a density of 930 kg/m3 or less and a polyethylene resin composition. In these films, however, the layer of a mere mixture of polypropylene resin and polyethylene resin is used as a heat-sealed layer and the films are not yet necessarily satisfactory in view of the balance between sealability and easy openability.
Particularly in recent years, propylene polymers have been heavily used as materials for packaging containers, in view of transparency, heat resistance, hygienic properties, strength, costs, etc. Moreover, package is sterilized by high temperature, depending on the kind of contents. For these reasons, heat sealable materials for having an easy-openable property are strongly required to ensure excellent balance between sealability and easy openability for propylene polymers as well as excellent heat resistance, namely, maintaining the sealing strength to withstand air expansion in the inside of packages upon sterilization processing at high temperatures and minimizing changes in the sealing strength caused by such high temperature processing. However, the materials proposed above fail to meet these requirements.
The present inventors made extensive studies to develop heat sealable materials having an easy-openable property, which satisfy all of the performance requirements described above, and especially for propylene polymers, which are excellent in sealability, easy openability and appearance when opening, have heat seal strength with less temperature dependency and minimize changes in sealing strength even by high temperature sterilization. As a result, it has been found that by the use of an ethylene polymer composition prepared by adding a specific ethylene/α-olefin random copolymer to a propylene polymer and an ethylene polymer, a heat sealable material having an easy-openable property, which is excellent in easy peelability from a propylene polymer layer, smooth feeling when opening, antiblocking properties, hygienic properties, film processability and lamination processability, can be obtained. Thus, the present invention has been attained.